Multifrequency plasma reactor

ABSTRACT

A multifrequency plasma reactor includes first, second and third power generators operably coupled to at least one of an upper and lower electrode for generating power signals. The plasma reactor further includes a controller for selectively activating the power generators according to an activation profile that results in the formation of a desirable narrow gap via in a semiconductor wafer. A method of generating a plasma in the reactor for etching the semiconductor wafer is also described by way of configuring the power generators according to various activation configurations during various phases of the etching process.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a plasma reactor and, in particular, toa multiple frequency plasma reactor in which the frequencies and thepowers associated therewith are individually controllable.

2. State of the Art

Semiconductor fabrication techniques are used to form integratedcircuits on wafers and frequently include plasma-assisted processes foretching materials from the semiconductor wafer. Such plasma etchingprocesses, also known as “dry etching,” are conventionally performed ina plasma reactor which utilizes radio frequency (RF) power generators toprovide power to one or more electrodes within a vacuum chambercontaining a gas at a predetermined pressure as defined by a specificprocess. The plasma reactor also includes a matching network forefficiently coupling power from the RF power generator to the electrodewithin the vacuum chamber.

Dry etching of a semiconductor wafer occurs within a vacuum chamber whenelectric fields between the electrodes within the vacuum chamber causeelectrons present in the gas within the vacuum chamber to initiallycollide with gas molecules. With time, the electrons gain more energyand collide with the gas molecules to form an excited or ionizedspecies. Eventually, a plasma is formed in which excitation andrecombination of the atoms with electrons within the plasma arebalanced. Highly reactive ions and radical species result in the plasmaand are used to etch materials from the semiconductor wafer. Electricand magnetic fields within the vacuum chamber are used to control theetching processes on the semiconductor wafer.

One conventional RF-powered plasma reactor is a single-frequency diodereactor. In a single-frequency diode reactor, RF energy isconventionally applied to the wafer table on which the semiconductorwafer is located with an electrode located above the wafer serving as agrounded electrode. In such an arrangement, the plasma forms above thewafer and the ions are accelerated downward, as a result of an electricfield formed between the plasma and the negatively charged wafer, intothe wafer to physically etch materials from the wafer. Differentfrequencies presented at the electrode cause different physicalphenomena in the plasma, which may or may not be desirable for aparticular semiconductor process.

Another conventional RF-powered reactor includes a dual-frequencyreactor which generally permits one RF frequency to be applied to afirst powered electrode located away from the wafer and whichpredominantly controls and powers the plasma. A second RF frequencyelectrode provides a bias to the wafer to control the potential (e.g.,sheath potential) between the second powered electrode and the plasma.Such a configuration generally assumes a capacitively coupledarrangement, which results in the formation of a self-induced DC bias tothe wafer. Dual-frequency systems generally permit higher ion densitiesin the plasma, which results in a higher ion flux into the wafer. Suchan approach significantly affects etch rates as a higher density of ionsgenerally induces a higher etch rate.

Yet another conventional RF-powered reactor includes a dual-frequencyreactor which applies two RF frequencies to a biasing electrode tocontrol the potential between the biasing electrode and the plasma.Another electrode is located away from the wafer and is coupled to areference potential, such as ground. The two frequencies typicallyperform separate functions, with one frequency dominating the ion energywhile the other frequency dominates the plasma energy.

Though various arrangements for providing power to the plasma of aplasma reactor have been described, each heretofore-describedconfiguration includes corresponding shortcomings. Therefore, thereexists a need for an improved configuration which provides for aflexible solution to the foregoing problems and deficiencies.

BRIEF SUMMARY OF THE INVENTION

A multifrequency plasma reactor and method of etching a semiconductorwafer is provided. In one embodiment, a plasma reactor includes first,second and third power generators which are coupled to correspondingupper and lower electrodes for generating power signals. The plasmareactor further includes a controller for selectively activating thepower generators according to an activation profile that results in theformation of a desirable narrow gap via on a semiconductor wafer.

In another embodiment of the present invention, a plasma reactorincludes a vacuum chamber which includes upper and lower electrodestherein. First, second and third power generators couple to the upperand lower electrodes, the power generators selectively activated by acontroller according to a specific activation profile.

In yet another embodiment of the present invention, a method ofgenerating a plasma in a plasma reactor for etching a semiconductorwafer during an etch process is provided. First, second and third powergenerators are configured and operated according to a first activationconfiguration during a first phase of the etch process. The powergenerators are reconfigured and operated according to a secondactivation configuration during a second phase of the etch process. In ayet further embodiment of the present invention, an etching method isprovided wherein first, second and third power signals are generated atupper and lower electrodes with the power generators being individuallyactivated to control the etching of the semiconductor wafer.

In yet another embodiment of the present invention, a method for etchinga semiconductor wafer is provided. A plasma reactor is provided whichincludes three power generators coupled to upper and lower electrodes. Acontroller selectively activates the power generators and, bycontrolling the power generators, the etching process is furthercontrolled.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, which illustrate what is currently considered to be thebest mode for carrying out the invention:

FIG. 1 is a schematic diagram of a capacitively coupled plasma reactorutilizing three power generators, in accordance with an embodiment ofthe present invention;

FIGS. 2A and 2B are cross-sectional diagrams illustrating the formationof narrow gap vias utilizing the plasma reactor configured according tovarious embodiments of the present invention;

FIGS. 3A-3D illustrate power configurations of the three powergenerators, in accordance with embodiments of the present invention;

FIG. 4 illustrates a variable duty cycle of the three power generatorsof the plasma reactor, in accordance with an embodiment of the presentinvention;

FIG. 5 is a cross-sectional diagram illustrating the formation of anarrow gap via utilizing the plasma reactor, in accordance with anotherembodiment of the present invention; and

FIG. 6 is a flow chart of a variable duty cycle configuration of aplasma reactor, in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, “semiconductor” includes all bulk semiconductorsubstrates including silicon, silicon-on-insulator (SOI),silicon-on-sapphire (SOS), silicon-on-glass (SOG), gallium arsenide(GaAs), and indium phosphide (InP), etc. A triple-frequency plasmareactor 10 for processing semiconductor devices is shown at FIG. 1. Afirst, or upper, power generator 12 is utilized to generate plasma 14within vacuum chamber 16. Plasma reactor 10 further includes a second,or lower, high-frequency power generator 18 and a third, or lower,low-frequency power generator 20 used to bias the substrate of wafer 22as located upon wafer table 24. In the present embodiment, plasmareactor 10 is a parallel plate reactor having an upper electrode 26 anda lower electrode 28. Additionally, power generators 12, 18 and 20 arecapacitively coupled via respective capacitors 30-34.

Upper power generator 12 may be configured to generate a variablypowered RF signal of, by way of example and not limitation, between 1and 2 kilowatts of power at a frequency of approximately 40 to 100megahertz. Additionally, lower high-frequency power generator 18 may beconfigured, by way of example and not limitation, to generate a variablypowered RF signal of approximately 1 to 2 kilowatts of power and operateat a frequency range of approximately 13.5 to 60 megahertz. Yet further,lower low-frequency power generator 20, by way of example and notlimitation, may be configured to generate a variably powered RF signalof approximately 1 to 2 kilowatts of power at an operational frequencyof approximately 1 to 13.5 megahertz. While specific frequencies andpowers have been identified as examples, other rules may be applied foridentifying frequencies and powers according to a specific process. Suchrules may include guidance for selecting a frequency for the lowerhigh-frequency power generator 18, namely that the lower high-frequencypower generator 18 operates at a frequency greater than three times thefrequency of the lower low-frequency power generator 20. Another rulemay include that the upper power generator 12 be configured to operateat a frequency of at least that of the lower high-frequency powergenerator 18.

Additionally, proper operation of a plasma reactor requires soundgrounding techniques. Grounding plates 36 are illustrated and groundingmay further take place through the use of a matchbox (not shown) or acounter electrode (not shown), the configuration and implementation ofwhich is appreciated by those of ordinary skill in the art. Generally, amatchbox matches the impedance with the chamber and the generator. Inshort, the matchbox matches the impedance on both sides of the generatorin order to minimize reflected power, which otherwise would result in anineffective coupling of power into the plasma.

The exact frequencies of operation for the power generators may beselected to correspond to internationally recognizedindustrial/scientific/medical (ISM) apparatus frequencies or the outputfrequencies of commercially available RF power supplies. Utilization ofa frequency in the VHF signal band for upper power generator 12 isdesirable as frequencies in this range are more effective than lowerfrequencies at breaking down etch gases into reactive radicals andinitiating a plasma. Furthermore, the required pressure within vacuumchamber 16 may be reduced through the use of such frequencies.Additionally, higher frequencies beyond the VHF signal band also becomemore expensive to generate and to couple into the plasma. Thetriple-frequency plasma reactor 10 may further include a controller 38operably and controllably coupled with power generators 12, 18 and 20.Controller 38 may be programmable and may control the power generatorsin both wattage and frequency and may be further responsive to aconfigured duty cycle which enables a reconfiguration of the operationof the power generators during a semiconductor wafer treatment process.

While embodiments of the present invention contemplate variousoperational parameters on the corresponding power generators, as definedherein, the term “inactive” or similar terminology as applied to a powergenerator includes the deactivation of the entire power generator andfurther includes the reduction in dominating power of a specific powergenerator. Therefore, in lieu of disabling or turning off a powergenerator, a reduction in power, for example, from one or more kilowattsto one or more hundreds of watts results in the same overall effectwhile allowing some beneficial effects from the continued operation,albeit at a reduced level, of various power generators.

FIGS. 2A and 2B are illustrative cross-sectional profiles of narrow gapvias which may be formed by the triple-frequency plasma reactor of thepresent invention. The cross-sectional illustrations are not to scaleand are presented herein for illustrative purposes only of the variousnarrow gap profiles attainable through various combinations of theexcitation of power generators 12, 18 and 20 of FIG. 1. In FIG. 2A, thecross-sectional view as illustrated results from the configuration oftriple-frequency plasma reactor 10 (FIG. 1) according to theconfiguration or power profile of FIG. 3A. In FIG. 3A, the generatorsignal 40 of upper power generator 12 (FIG. 1) is inactive whilegenerator signal 42 of lower high-frequency power generator 18 (FIG. 1)and generator signal 44 of lower low-frequency power generator 20(FIG. 1) are set to active or defined levels. Such a configurationresults in a profile of a narrow gap via 46 which is directionallyetched as defined by a mask 48 through, for example, a glass or otherinsulative layer 50 to a contact or target layer 52. It should be notedthat narrow gap via 46 assumes a bowed profile as a result of, forexample, polymer buildup around the throat of the via.

The configuration or power profile of FIG. 3A provides processingbenefits including good mask or photoresist selectivity (i.e., the maskendurance through the plasma bombardment is relatively robust). Anotherbenefit of the present configuration is that resultant narrow gap viasexhibit a desirable relatively large opening at the bottoms thereof. Thepresent configuration further exhibits some less desirablecharacteristics, namely the bowing nature that occurs in the uppersection of the narrow gap via as a result of the constriction at thethroat portion or upper portion of the via.

In FIG. 2B, the cross-sectional view as illustrated results from theconfiguration of triple-frequency plasma reactor 10 (FIG. 1) accordingto the power profile of FIG. 3B. In FIG. 3B, the generator signal 54 ofupper power generator 12 (FIG. 1) is set to active for a defined levelwhile generator signal 56 of lower high-frequency power generator 18(FIG. 1) is inactive. Furthermore, generator signal 58 of lowerlow-frequency power generator 20 (FIG. 1) is set to an active or definedlevel. Such a configuration results in a profile of a narrow gap via 60which is directionally etched as defined by a mask 62 through, forexample, a glass or other insulative layer 64 to a contact or targetlayer 66. It should be noted that narrow gap via 60 assumes a taperednarrowing profile as the depth through insulative layer 64 increases.

The configuration or power profile of FIG. 3B provides processingbenefits including a good initial profile at the throat or top of thenarrow gap via. The present configuration further exhibits some lessdesirable characteristics, namely the appreciable narrowing of the viaas the depth into the via increases. Therefore, the contact area at thebottom of the via must be accounted for with the depth and initialopening size at the top of the via.

FIG. 3C and FIG. 3D represent other configurations of excitation ofpower generators 12, 18 and 20 of the triple-frequency plasma reactor10. Specifically, in FIG. 3C, the generator signal 68 of upper powergenerator 12 (FIG. 1) is set to an active or defined level as is thegenerator signal 70 of lower high-frequency power generator 18 (FIG. 1).Generator signal 72 of lower low-frequency power generator 20 (FIG. 1)is inactive. Referring to FIG. 3D, all frequencies 74, 76 and 78 are setto active or defined levels for creation of plasma as used in a specificdry etching semiconductor process.

FIG. 4 is a power profile of the excitation of the respective powergenerators of the triple-frequency plasma reactor 10, in accordance withanother embodiment of the present invention. The previous embodimentshave illustrated a static configuration of the various power generatorsof the triple-frequency plasma reactor and the corresponding narrow gapvias resulting therefrom. In the present embodiment, a dynamicexcitation of power generators 12, 18 and 20 is illustrated by way ofthe formation of duty cycles associated with each of the powergenerators. Those of ordinary skill in the art appreciate that narrowgap vias are typically formed for the further formation of an electricalconnection through the via to the corresponding target layer, such as aconductive trace or a pad. Ideally, the formation of a narrow gap viahaving sidewalls perpendicular with the target layer and with anadequate aspect ratio for accommodating a reliable filling of the narrowgap via is desirable. However, as previously illustrated in FIGS. 2A and2B, various profiles of narrow gap vias exhibit desirable andundesirable profile characteristics.

Formation of a narrow gap via occurs as the plasma etching processproceeds over a continuum of time as defined by an etch rate and aresulting profile. The present embodiment varies the excitation of thepower generators to advantageously formulate the plasma and theresulting electrical fields to select desirable etching characteristicsover an entire etching process. In FIG. 4, various duty cycles aredefined for the respective frequencies. In a first phase 80, generatorsignal 82 of upper power generator 12 (FIG. 1) is set to an active ordefined level. Generator signal 84 of lower high-frequency powergenerator 18 (FIG. 1) is inactive. Additionally, generator signal 86 oflower low-frequency power generator 20 (FIG. 1) is also set to an activeor defined level during first phase 80. The first phase configuration ofpower generators 12, 18 and 20 of the triple-frequency plasma reactor 10(all of FIG. 1) similarly corresponds to the configuration asillustrated above with regard to FIG. 3B and correspondingly with theformation of an acceptable initial opening of narrow gap via 60 of FIG.2B. Correspondingly, the narrow gap via 88 of FIG. 5 illustrates theformation of an initial opening during first phase 80.

Returning to FIG. 4, a second phase 90 alters the excitation of powergenerators 12, 18 and 20 in an arrangement wherein generator signal 82of upper power generator 12 (FIG. 1) is inactive while generator signal84 of lower high-frequency power generator 18 (FIG. 1) and generatorsignal 86 of lower low-frequency power generator 20 (FIG. 1) are set toactive or defined levels. Such a configuration results, during secondphase 90, of a more widened profile than would otherwise be attainablethrough the previous configuration as illustrated with reference tofirst phase 80. Such a resulting narrow gap via profile is illustratedwith reference to FIG. 5. As a large aperture is desirable when matingwith a target layer, such as target layer 92 of FIG. 5, areconfiguration of the excitation of power generators 12, 18 and 20 isdesirable. With reference to FIG. 4, a third phase 94 reconfigures theexcitation in a manner consistent with the excitation of first phase 80,namely generator signals 82 and 86 of upper power generator 12 and lowerlow-frequency power generator 20, respectively, are set to active ordefined levels while generator signal 84 of lower high-frequency powergenerator 18 (FIG. 1) is inactive. Such a configuration of theexcitation of the corresponding power generators enables the formationof a more desirably larger aperture when coupling with target layer 92.

FIG. 6 is a flowchart of a variable duty cycle multiple frequency plasmareactor, in accordance with an embodiment of the present invention. InFIG. 6, the power generators are configured 100 for a first phase. Thestatus or completion of the first phase is queried 102 until thecompletion of the first phase. The power generators are reconfigured 104for a subsequent phase with the duration of that phase queried 106 untilits completion. Upon its completion, the determination of the last phaseis queried 108 with any remaining phases being reconfigured 104 untileach phase is completed.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the inventionincludes all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

1. A parallel plate plasma reactor, comprising: first, second and thirdpower generators wherein the first power generator is capacitivelycoupled to an upper electrode and the second and third power generatorsare capacitively coupled to a lower electrode for supporting a waferthereon, the first, second and third power generators beingfrequency-based power generators; and a controller configured toindividually selectively activate the first, second and third powergenerators to a plurality of activation configurations during aplurality of phases of a duty cycle of a process, wherein at least oneof the plurality of activation configurations includes differentlyactivating the second and third power generators to generate at leasttwo different active states on the lower electrode; wherein each of thesecond and third power generators are configured to independentlyreceive a signal from the controller and independently apply energydirectly to the lower electrode entirely to generate the at least twodifferent active states thereon.
 2. The plasma reactor of claim 1,wherein the second power generator is configured to operate at afrequency of at least three times an operational frequency of the thirdpower generator.
 3. The plasma reactor of claim 1, wherein the firstpower generator is configured to operate at a frequency of at leastgreater than or equal to each of an operational frequency of the secondpower generator and an operational frequency of the third powergenerator.
 4. The plasma reactor of claim 1, wherein the controller isoperable to place the first power generator in an inactive mode and thesecond and third power generators in an active mode.
 5. The plasmareactor of claim 1, wherein the controller is operable to place thefirst and third power generators in an active mode and the second powergenerator in an inactive mode.
 6. The plasma reactor of claim 1, whereinthe controller is operable to place the first and second powergenerators in an active mode and the third power generator in aninactive mode.
 7. The plasma reactor of claim 1, wherein the controlleris operable to place the first, second and third power generators in anactive mode.
 8. The plasma reactor of claim 1, wherein the controllerduring a process is operable to configure the first, second and thirdpower generators to a first activation configuration during a firstphase thereof and to reconfigure the first, second and third powergenerators to a second activation configuration during a second phasethereof.
 9. The plasma reactor of claim 1, wherein the controller isfurther operable to control power levels of the first, second and thirdpower generators during the plurality of activation configurations. 10.The plasma reactor of claim 1, wherein each of the first, second andthird power generators is capacitively coupled to one of the upper andlower electrodes.
 11. The plasma reactor of claim 1, wherein the secondpower generator operates at a frequency of about 13.5 MHz to about 60MHz.
 12. The plasma reactor of claim 1, wherein the first powergenerator operates at a frequency of about 40 MHz to about 100 MHz. 13.The plasma reactor of claim 1, wherein the third power generatoroperates at a frequency of about 1 MHz to about 13.5 MHz.
 14. A parallelplate plasma reactor, comprising: a vacuum chamber including upper andlower electrodes therein; first, second and third power generatorswherein the first power generator is capacitively coupled to an upperelectrode and the second and third power generators are capacitivelycoupled to a lower electrode for supporting a wafer thereon, the first,second and third power generators being frequency-based powergenerators; and a controller configured to individually selectivelyactivate the first, second and third power generators to a plurality ofactivation configurations during a plurality of phases of a duty cycleof a process, wherein at least one of the plurality of activationconfigurations includes differently activating the second and thirdpower generators to generate at least two different active states on thelower electrode; wherein each of the second and third power generatorsare configured to independently receive a signal from the controller andindependently apply energy directly to the lower electrode entirely togenerate the at least two different active states thereon.
 15. Theplasma reactor of claim 14, further comprising a wafer table, whereinthe lower electrode is coupled to the wafer table and the upperelectrode is arranged above the wafer table.
 16. The plasma reactor ofclaim 14, wherein each of the first, second and third power generatorsis capacitively coupled to one of the upper and lower electrodes. 17.The plasma reactor of claim 14, wherein the first power generator iscapacitively coupled to the upper electrode and the second and thirdpower generators are capacitively coupled to the lower electrode. 18.The plasma reactor of claim 17, wherein the second power generator isconfigured to operate at a frequency of at least three times a frequencyof the third power generator.
 19. The plasma reactor of claim 18,wherein the second power generator is configured to operate at afrequency of about 13.5 MHz to about 60 MHz.
 20. The plasma reactor ofclaim 18, wherein the first power generator is configured to operate ata frequency of about 40 MHz to about 100 MHz.
 21. The plasma reactor ofclaim 18, wherein the third power generator is configured to operate ata frequency of about 1 MHz to about 13.5 MHz.